Many industrially important aromatic nitrations, such as the production of nitrobenzene and dinitrotoluene, are commonly carried out in mixed acid, a nitrating solution of sulfuric acid and nitric acid. There are a number of different commercially practiced mixed acid nitrations. In the case of nitrobenzene manufacture, adiabatic nitration is commonly used (U.S. Pat. Nos. 4,091,042, 5,313,009, 5,763,697). Dinitrotoluene manufacture is carried out in concentrated mixed acid under isothermal and adiabatic conditions (U.S. Pat. Nos. 5,902,910, 5,689,018, 4,496,782).
Regardless of the type of aromatic or operating conditions, a key performance characteristic of mixed acid nitration processes is the amount and type of nitro-hydroxy-aromatic byproducts produced. For example, nitration of benzene produces nitrophenols, nitration of toluene produces nitroresols, and nitration of xylene produces nitroxylenols. The majority of nitrated aromatics are subsequently hydrogenated to their corresponding amine. It is common practice to remove nitro-hydroxy-aromatic byproducts from the nitroaromatic product, as these compounds are believed to adversely affect hydrogenation catalyst performance. Nitro-hydroxy-aromatic byproducts are commonly extracted from the crude nitroaromatic into alkaline wash water through counter-current washing. A number of different alkali chemicals are commonly used in washing, caustic soda and aqueous ammonia being the most common.
Treatment of the alkaline nitro-hydroxy-aromatic contaminated wash water, commonly called redwater due to its characteristic color, remains an active area of research and development. In the quantities and concentrations generated commercially, these compounds are toxic to most biological wastewater treatment systems. Due to its toxicity, nitration redwater is typically segregated from other plant effluents for separate treatment, usually in a chemical or thermal treatment process, leaving the majority of the nitration effluent relatively free of nitro-hydroxy-aromatic compounds. In order to reduce the quantity of contaminated effluent, and thus the cost of the special treatment required, alkaline wash water flow is commonly kept to the minimum required to achieve adequate product purity. Separation of inorganic compounds from the redwater is also important for some treatment technologies. For example, many sulfur compounds will produce sulfur dioxide upon incineration and, therefore, redwater should contain low levels of sulfate if it is to be incinerated.
No single redwater treatment technology has achieved widespread adoption by the nitration industry. The choice of a treatment technology is often dependent on local site conditions, economics and operator preference. The more common treatment techniques used commercially, and some previously disclosed processes for treatment of nitration redwater include:
Direct incineration of redwater. In this case, ammonia is commonly used as the alkali rather than caustic soda, since no ash is produced during combustion. However, incinerators can be unreliable due to high temperature operation and additionally have high operating costs due to the large amount of fuel required to first evaporate the redwater and then to raise it to combustion temperatures. Incineration is limited to ammonia washing since sodium or potassium hydroxide would react with carbon dioxide to form solid or molten carbonate salts in the incinerator. Incineration also often suffers from poor public image. PA1 High-pressure, subcritical thermal-treatment. This technique includes wet air oxidation, pressurized thermal treatment without addition of an oxidant (U.S. Pat. No. 4,230,567), and thermal treatment with the addition of an oxidant such as nitric acid (U.S. Pat. No. 5,232,605) or oxygen (U.S. Pat. No. 5,250,193). These processes are effective at detoxification of redwater, but, due to the formation of stable intermediates such as carboxylic acids, further biological treatment is commonly required before release to a receiving water. These processes typically embody large, high-pressure reactors under exothermic conditions, posing some safety concerns. PA1 High temperature, alkaline digestion of redwater. U.S. Pat. No. 5,221,440 describes an alkaline digestion process whereby nitro-hydroxy-aromatic compounds are converted to less toxic, oxalic acid-like compounds. The resulting effluent must then be neutralized and biologically treated before discharge. PA1 Biological treatment. By blending the redwater with another much larger wastewater stream, the nitro-hydroxy-aromatic toxicity can be reduced to non-toxic levels. However, there remains the possibility of toxic shock to the biological system if nitro-hydroxy-aromatic concentrations are too high. In addition, biological removal efficiencies can be too low to meet discharge standards and commonly require the use of activated carbon or another secondary treatment method. PA1 Acid precipitation, extraction and incineration. Nitro-hydroxy-aromatic compounds, such as nitrophenols or nitrocresols, can be precipitated under acidic conditions. The precipitates can be extracted from the aqueous phase into an organic phase for subsequent incineration (U.S. Pat. Nos. 4,597,567 and 4,925,565). Although these processes allow incineration of a concentrated nitro-hydroxy-aromatic solution, thereby reducing fuel costs, these processes still suffer from incinerator permitting issues. Even after the multiple unit operations of acidification, precipitation, extraction, concentration and incineration, the redwater extract typically requires further treatment to meet discharge standards. PA1 Chemical oxidation. Redwater treatment processes using ozone, hydrogen peroxide/fentons reagent (U.S. Pat. No. 4,604,214) and iron/hydrochloric acid (U.S. Pat. No. 4,708,806) are known. Achieving required discharge standards with these processes without further processing is difficult and chemical reagent costs are high. PA1 Activated carbon. Treatment of industrial scale redwater waste with activated carbon is technically feasible, but is prohibitively expensive. PA1 Deep well injection. Some facilities dispose of redwater through deep disposal wells. This is not acceptable practice in most jurisdictions and does not reduce the toxicity of the effluent.
It is clear, therefore, that there remains a need for an effective redwater treatment process, especially for nitration production facilities that do not have access to a biological treatment plant. In addition, currently practiced redwater treatment technologies are not integrated into the nitroaromatic washing process. Integration provides the opportunity to recover and recycle water, product and chemicals.